The present invention relates to apparatus and method for fluid measurement and is particularly applicable to the measurement of individual proportions of the component phases of a multiphase fluid.
It is particularly useful in the field of fluid flow from oil and gas wells. The fluid flowing from a well generally comprises a mixture of oil, gas and water and it can be of great advantage to measure the individual phases without first having to separate them.
This has been done using apparatus as disclosed in EP-A-0424 503. The relative amount of water and oil is often critical but unfortunately inherently difficult to measure accurately when there is a large amount of gas in the mixture.
The present invention seeks to improve the accuracy of fluid flow measurements.
According to the present invention there is provided a method of measuring the relative proportions of oil and water in a flowing multiphase fluid, the method comprising allowing the gas, oil and water phases of the multiphase fluid at least partially to separate, diverting at least some of the separated gas phase from the flowing fluid, into a bypass conduit, measuring the relative proportions of oil and water in the undiverted fluid.
Preferably the multiphase fluid is directed to flow through a separating module wherein the gas phase tends to separate and collect above a pool of fluid.
In a particularly preferred embodiment such a separating module comprises the tank of a homogenising unit. Such as is described in EP-A-0 379 319. The bypass conduit is then constructed with an inlet leading from the upper section of the tank (where gas collects) and an outlet joining the outlet pipe leading from the homogeniser, downstream of metering apparatus. A valve is located in the bypass conduit.
The valve may be manually or automatically operated, for example based on a pre-programmed metering cycle. The valve may be a choke or alternatively arranged to allow a flow rate for the bypassing gas phase appropriate to the pressure drop monitored by the multiphase flow meter located in the outlet pipe from the homogenising unit.
The valve will be kept closed for normal operation, but will be opened periodically for the performance of high quality water-oil ratio measurements. Normally the water-oil ratio does not vary dramatically over short periods of time. The high quality measurement made using the method of the present invention can be compared to normal measurements for improved accuracy or to update calibration values or parameters.
The valve may also be remotely operated.
The method of the invention also allows in-situ sensor calibration operations to be performed. This is particularly useful for example for sub-sea applications and other permanent installations where there is an unknown mixture in the meter, so that calibration was hitherto very difficult.
Using the method of the invention, calibration can be effected at two different conditions, such as two different gas volume fractions, for the same production conditions.
No measurement of the fluid in the bypass conduit is necessary and it is not necessary to ensure that only the gas phase is diverted; spillover of liquid phase will not affect operation of the invention.
Apparatus for performing the method is also provided.
Thus the invention provides a very versatile method of increasing the accuracy of fluid flow measurements on multiphase fluids.